PV Assistant Bug Causes Battery Voltage Overshoot

DIY
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I am running an off-grid installation with Multiplus II 48/5000, MPPT chargers and AC out coupled PV, using PV Assistant.

The AC coupled PV inverter is a SMA Sunny Boy 3000-TL21. Its frequency response is a linear reduction in output between 51Hz and 52Hz. These are the parameters I gave when setting up the ESS Assistant. See second screenshot.

The first screenshot shows what happens on a typical day with low AC loads when the battery reaches the target charge voltage of 55.2V.

Starting at 9:59 battery charge current is gradually reduced by reducing power from the MPPT chargers. Battery voltage remains almost constant just a tad over 55.2V during this time, as it should.

Then once the MPPTs are at zero output, from about 10:01 to 10:03 charge current remains constant (sourced from the AC coupled PV). That leads to an overshooting in battery voltage all the way to 55.6V. During that time the Mutliplus slowly increases frequency from 50Hz to 51Hz.

The Multiplus knows that any frequency changes between 50 and 51Hz will have no effect. When setting up the PV assistant it was told: output reduction starts at 51Hz. This is as designed, no fault of the Sunny Boy.

Once the frequency gets over 51Hz the AC inverter output is reduced and the battery voltage slowly returns to normal.

If I was charging to 3.6V cell voltage (57.6V battery voltage, as often is done for LFP), this same overshooting would trip the BMS, causing a completely unnecessary power outage. It is only my conservative 3.45V target cell voltage that leaves enough buffer.

This is a major bug in the PV assistant. Once the MPPT chargers are effectively turned off, with still a surplus of charge current, the Multiplus frequency should instantly jump to 51Hz and then slowly rise from there. That would cut out the 3 minutes of constant current that cause the over-voltage.

There is absolutely no benefit in slowly moving from 50Hz to 51Hz.

Interesting enough: when the battery target-voltage changes to float (53.6V) at about 9:08, the frequency does jump to 52Hz a lot quicker, reducing output from the AC coupled inverter to zero in an instant. That actually is a nuisance, since there is no need to rush to get to float.

It is worth mentioning that the target voltage is set by the BMS via CAN. A couple of reasons to use close-coupled communication. First: seeing things like this, I actually do trust the BMS to do a better job in protecting the battery than the Victron ecosystem. But also: the BMS allows a much finer grained adjustment, i.e. just 18 minutes absorption, while Victron would force a 1 hour minimum on the battery.

Edit: why are the MPPT controllers tuned down to zero before the frequency response happens on the AC side? Would it not make sense to ramp both of them down at the same time, or even to start ramping down the AC coupled PV first, to reduce the load going through the Multiplus?

p.s.: ESS Assistant has the same bug, which has exactly the same effect when the grid is disconnected.

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Hi @P_e_t_e_r,

The SMA SB can be programmed to be more or less sensitive, it has many adjustable settings to work in different AC Coupled systems.

Given the behavour and ramp speed of the frequency curve is not adjustable on the Victron side it needs to be changed on the SMA side.

It has been a while since I have programmed a SunnyBoy - I believe these are still the up to date instructions on how to do it - Changing Operating Parameters

The Sunnyboy (SB) will need to be set to OFF GRID MODE.

The parameters themselves are listed here - https://files.sma.de/downloads/NG_PAR-TB-en-22.pdf

OffGri.HzdStr - ”Start point of the power control via frequency”
OffGri.HzdLim - ”Endpoint of the power control via frequency”

The values you can set for each parameter are between 0…5

I’d recommend a linear reduction of PV power between 50.2 Hz and 52.7 Hz as this best suits both devices. But you can adjust it further if you’re still having issues.

That is -

OffGri.HzdStr = 0.2
OffGri.HzdLim = 2.7

The parameters themselves are arbitrary and just need to match the characteristics and settings of both Victron and SMA devices.
The SMA default is for use with their SMA inverter chargers.

A lot of the reliability and stability of AC coupled PV depends on the size and voltage characteristics of the battery.

Seeing as you have DC MPPT chargers as well, it might just be simpler to reduce the target charge voltage of the MultiPlus so it finishes charging sooner and begins to increase AC frequency while the more responsive DC MPPT solar chargers do the finishing part of the charge that is causing the issues.

Which approach you prefer is up to you.

1 Like
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Thank you for giving options.

A couple of questions to clarify details:

  1. In order to set different voltages in Multiplus and MPPT, would that mean I need to disable DVCC and also BMS control of the target voltage? Or does the Multiplus still use the parameters given, even with DVCC enabled? I did set the parameters as per screenshot, even though I fully expected they would not be used…

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  2. If Victron does not take the minimum frequency for power adjustment into account, why is this one of the parameters asked for when setting up PV Assistant?

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Last night I changed the parameters exactly as suggested. Both for the SB3000 and in VE.Configure.

I did turn the SMA inverter off and back on (waiting a couple of minutes in between to ensure it was actually off). The parameters were saved (I saw them still after the reboot). But output stays unchanged until frequency rises over 51Hz.

So there we have it, a bug in the SMA inverter. I doubt that SMA will do anything about it, it is a 12 year old model, this unit was first installed 9 years ago. Long discontinued. I got it second hand. And it works according to its specification. It just does not allow changes.

On the other hand Victron still sells the Multiplus II 48/5000. I just bought it brand new a couple of months ago. So I reckon it is up to Victron to fix this bug: make sure the parameter given for start frequency for output reduction (in my case 51Hz) is actually taken into account when adjusting frequency!

Because: why ask for it during configuration, to then completely ignore it? This is a bug in PV assistant.

As a side note: this morning charge current was over 60A. I manually reduced the DVCC current limit to 30A to slow down charging and get the battery to full when I had time to monitor what happens.

In this case the required frequency change happened in a matter of seconds and very effectively curtailed output from the Sunny Boy, bringing the charge current in line with the 30A limit.

That confirms that there are no technical reasons why the Multiplus can not adjust frequency quickly. It does so in response to over-current. But it chooses to be slow in response to over-voltage.

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Kia ora and thanks @guystewart for the references, @P_e_t_e_r I have a similar setup to you (EasySolar includes MultiPlus II and SmartSolar MPPT), and find it excellent for daytime consumption (work from home and AC charge EVs, for which PV Inverter solar-to-grid is much more efficient than EasySolar solar-to-battery-to-grid) :
I have a 2nd hand SB3000HF, set in my case to “SMA Island Mode 50 Hz” and no-one wants to change that for me.
Using the maintainer password (it came to me set to the default) I have set
”Start point of the power control via frequency 0.20 Hz
End point of the power control via frequency 1.50 Hz”
I think I reduced the end point voltage offset because when it was higher, some appliances were unhappy- washing machine, maybe breadmaker. You cannot even see these settings without either the maintainer password, or the RS485-based configuration software.

I am having reasonable results with these matching PV Inverter Assistant settings:

PV Inverter support (size:799)
*) Restart PV Inverters when the (temperature compensated) battery voltage becomes lower than 55.50 V.
*) The solar converter will start reducing its output power at 50.20 Hz.
Output power will be reduced to minimum when the frequency is 51.30 Hz.
The converter will disconnect when the frequency is higher than 51.50 Hz.
*) Total installed PV inverter power is 3000 Watts.
Total installed PV panel power is 3000 Watts.

I don’t have DVCC and I’m reasonably sure I read that it is not compatible with the PV Inverter Assistant.

I had a significant voltage overshoot after I upgraded the MultiPlus’ software before I realised that it came back up without the Assistant.

My only gotcha at the moment is that when the Multiplus raises the AC frequency to 51.5 Hz, it tends to stick there longer than I think reasonable- often overnight, and I am off grid. The “Restart PV Inverters” voltage setting seems to conflict with the online docs https://www.victronenergy.com/live/assistants:pv-inverter-support which indicates it is a fixed offset from the Float voltage.

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Thanks for your reply Graham. Looks like your SB3000HF doesn’t have the same software bug that my SB3000-TL21 has and actually follows the changed parameters. Lucky you!

I found a workaround. I am using Home Assistant to monitor and now control a lot of things around my house. I’ve put it to good use with this situation: By looking at battery SOC and voltage I managed to set up some rules in Home Assistant that turn down the DVCC maximum charge current long before there is any danger of overshooting the target voltage. That limitation in DVCC current kickstarts the process of raising the MultiPlus frequency well before it becomes an issue. By the time the set battery voltage is reached, the frequency already is in the ‘responsive’ part of the range.

A couple of months ago I added a Smart Shunt to my setup. That further improved things by providing a very reliable SOC value.

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Smart, apologies I couldn’t deliver a better solution but appreciate you following up with yours.

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Thanks Guy.

If others come across this thread with a similar situation, it is probably worth mentioning that Node Red running on the Cerbo should be able to achieve the same without the need for Home Assistant. Doing that would be slightly more robust too since it doesn’t need to communicate via a network with external hardware.

For me Home Assistant was just easier, since I use it on a daily basis, while Node Red is still on the ‘should learn to use it some day’ list.

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Thanks @P_e_t_e_r

My SunnyBoy has only a Windows UI via Bluetooth. There is some automation possible via RS485 but I have not gone there yet.

I also use Home Assistant -excellent- but I have not yet got stuck into automation.

I have a new (2 years now) with LED lighting throughout, with both daylight and warm which I can switch manually via HA. My first automation is going to be favouring the daylight component during the day and blending to the warm component through evening. But it’s still on the “when I retire” list of things to do.

Best, Graham